Ultra-wide band (UWB) ground-penetrating RADAR technology can be used for a number of applications, such as finding mines buried in the ground. In such applications, a transmitting antenna directs an impulse toward the ground. A receiving antenna then receives a direct wave from the transmitting antenna, followed by a reflected wave from the ground. The reflected wave is typically sampled, digitized, stored, and analyzed to determine the electrical properties, and hence the material content, of the underlying ground. The outgoing impulse may be in the order of 50-1,000 picoseconds wide, with meaningful energy content from below 1 GHz up to 20 GHz or more. In these applications, the received signal must be captured with high resolution in order to distinguish subtle changes in the reflected wave.
One approach to digitizing the UWB pulses is the use of equivalent time (ET) sampling to capture the received signal. In this method, the outgoing pulse is transmitted many times, and the reflected wave is sampled and digitized once per pulse at a specific delay setting from the timing of the transmitted pulse. Multiple samples are taken at each delay setting to allow for averaging of the samples to reduce noise, and multiple delay settings are used to form an equivalent picture of the change in the reflected voltage versus the time of flight. For example, seven samples may be averaged at each delay setting, and 1,500 delay settings spaced in 10 picosecond increments may be used to capture a 15 nanosecond window of the reflected wave at an equivalent 100 Giga-Samples per second (GS/s).
ET sampling systems only need to sample once per pulse allowing such systems to use a relatively high-resolution and low-cost digitizer. However, ET systems also have a number of significant drawbacks. Firstly, acquisition time is long. For example, with regard to the example described above, 10,500 pulses would be required to acquire one reflected waveform. Secondly, if the transmitted pulses are randomized in time, e.g., spread in spectrum, to avoid detection or interference with communications, the sampling time must be similarly randomized. Any mismatch between these two will add jitter to the ET sampling process. Also, coherent interfering signals, e.g., from communication equipment, become random in nature when sampled at the rate of outgoing pulses, and thus become more difficult to recognize and remove from the data algorithmically.
A real-time (RT) sampling and digitization process is generally preferred over an ET system, but a high-resolution, high sample rate RT digitizer is quite costly. For example, a 10-bit, 30 to 50 GS/s digitizer would be needed to implement a full RT system instead of the ET system described above.
Accordingly, a need remains for improved ground-penetrating RADAR and return signal analysis systems, particularly with regard to identifying materials below ground such as buried mines.